This invention relates to the field of ultrasound imaging systems. In particular, the present invention relates to field deployable 3D ultrasound imaging systems providing high resolution images in real time.
Ultrasound sensing and imaging technology provides a powerful tool for non-invasive imaging for treatment assessment and for minimally invasive surgery. Unlike CT scanners and MRI ultrasound imaging systems are compact and much cheaper to manufacture. These advantages allow use of ultrasound imaging systems in mobile units such as an ambulance or a helicopter. In general, victims of accidents, disasters or wars need immediate assessment and treatment in order to save their lives. For example, deployment of compact ultrasound imaging systems in mobile units allows on site imaging for treatment assessment during transportation providing live saving information for later surgery in a hospital or even providing information for minimally invasive surgery within the mobile unit. Therefore, it would be highly advantageous to provide a compact field-deployable 3D ultrasound imaging system for mobile units and field hospitals for immediate imaging of victims on accident sites, in disaster areas or in war zones.
However, state of the art ultrasound imaging systems suffer from very poor image resolution due to the very small size of sensor arrays of compact systems. Therefore, such systems do not provide images having a satisfying resolution for treatment assessment or surgery. In order to improve image quality it is necessary to deploy a large number of sensors in a compact multidimensional array to provide significant improvements in array gain for signals embedded in partially correlated noise fields. Partially correlated noise fields are caused, for example, by non-linear propagation characteristics of the human body and result in aberration effects and fuzziness in reconstructed images. The improvements in array gain result in image resolution improvements and minimization of the aberration effects.
An overview of the state of the art in adaptive and synthetic aperture beamformers is given in xe2x80x9cImplementation of Adaptive and Synthetic Aperture Processing Schemes in Integrated Active-Passive Sonar Systemsxe2x80x9d, Proceedings of the IEEE, 86(2), pp. 358-397, February, 1998 by S. Stergiopoulos. These algorithms have been designed to increase the signal-to-noise ratio for improved target detection and to provide simultaneously parameter estimates such as frequency, time delay, Doppler shift and bearing for incorporation into algorithms localising, classifying and tracking acoustic signals.
To optimize the beam forming process, beamforming filter coefficients have to be chosen based on data received from a sensor array of the sonar system. In particular, the coefficients have to be chosen based on the statistical properties of the received data. Algorithms using characteristics of noise received from the sensor array for optimizing the beamforming process are called adaptive beamformers. The adaptive beamformers require knowledge of a correlated noise""s covariance matrix. However, if the knowledge of the noise""s characteristic is inaccurate, performance of the adaptive beamformer will degrade significantly and may even result in cancellation of a desired signal. Therefore, it is very difficult to implement useful adaptive beamformers in real time operational systems. Numerous articles on adaptive beamformers and the difficulties concerning their implementation have been published. Various adaptive beamformers such as the Generalized Sidelobe Cancellers (GSC), the Linearly Constrained Minimum Variance Beamformers (LCMV) and the Minimum Variance Distortionless Response (MVDR) are discussed in the following references, which are hereby incorporated by reference:
B. Windrow et al.: xe2x80x9cAdaptive Antenna Systemsxe2x80x9d, Proceedings IEEE, 55(12), pp. 2143-2159, 1967;
N. L. Owsley: xe2x80x9cSonar Array Processingxe2x80x9d, S. Haykin, Editor, Prentice-Hall Signal Processing Series, A. V. Oppenheim Series Editor, pp. 123, 1985;
B. Van Veen and K. Buckley: xe2x80x9cBeamforming: a Versatile Approach to Spatial Filteringxe2x80x9d, IEEE ASSP Mag., pp. 4-24, 1988;
J. Capon: xe2x80x9cHigh Resolution Frequency Wavenumber Spectral Analysisxe2x80x9d, Proc. IEEE, 57, pp. 1408-1418, 1969;
S. Haykin: xe2x80x9cAdaptive Filter Theoryxe2x80x9d, Prentice-Hall, Englewood Cliffs, N.J., 1986;
T. L. Marzetta: xe2x80x9cA New Interpretation for Capon""s Maximum Likelihood Method of Frequency-Wavenumber Spectra Estimationxe2x80x9d, IEEE-Trans. Acoustic Speech Signal Proc., ASSP-31(2), pp. 445-449, 1983;
A. H. Sayed and T. Kailath: xe2x80x9cA State-Space Approach to Adaptive RLS Filteringxe2x80x9d, IEEE SP Mag., pp. 18-60, July, 1994;
A. B. Baggeroer, W. A. Kuperman and P. N. Mikhalevsky: xe2x80x9cAn Overview of Matched Field Methods in Ocean Acousticsxe2x80x9d, IEEE J. Oceanic Eng., 18(4), pp. 401-424, 1993;
H. Wang and M. Kaveh: xe2x80x9cCoherent Signal-Subspace Processing for the Detection and Estimation of angles of Arrival of Multiple Wideband Sourcesxe2x80x9d, IEEE Trans. Acoust. Speech, Signal Proc., ASSP-33, pp. 823-831, 1985;
J. Krolik and D. N. Swingler: xe2x80x9cBearing Estimation of Multiple Brodband Sources using Steered Covariance Matricesxe2x80x9d, IEEE Trans. Acoust. Speech, Signal Proc., ASSP-37, pp. 1481-1494, 1989;
S. D. Peters: xe2x80x9cNear-Instantaneous Convergence for Memoryless Narrowband GSC/NLMS Adaptive Beamformersxe2x80x9d, submitted to IEEE Trans. Acoust. Speech, Signal Proc., January 1995;
L. J. Griffiths and C. W. Jim: xe2x80x9cAn Alternative Approach to Linearly Constrained Adaptive Beamformingxe2x80x9d, IEEE Trans. on Antennas and Propagation, AP-30, pp. 27-34, 1982; and,
D. T. M. Slock: xe2x80x9cOn the Convergence Behavior of the LMS and the Normalized LMS Algorithmsxe2x80x9d, IEEE Trans. Acoust. Speech, Signal Proc., ASSP-31, pp. 2811-2825, 1993.
Unfortunately, implementation of adaptive beamformers in modern ultrasound systems comprising multi-dimensional arrays with hundreds of sensors requires very large amounts of memory and very large processing capabilities for real time data processing making their application for field-deployable systems impossible. To implement adaptive beamformers using current computer technology, the concept of partially adaptive beamformer design has been developed. The partially adaptive beamformer reduces the number of degrees of freedom, associated with the beamforming process, lowering the computational requirements and improving response time. Unfortunately, due to the reduction of the number of degrees of freedom the partially adaptive beamformers cannot converge to an optimum solution as fully adaptive beamformers. Therefore, the partially adaptive beamformers cannot make substantial use of multidimensional arrays deployed in ultrasound systems in order to improve array gain and to provide images with high resolution.
It is, therefore, an object of the invention to overcome the problems associated with the implementation of adaptive beamformers in modem ultrasound imaging systems comprising multidimensional sensor arrays.
It is further an object of the invention to provide adaptive beamformers with near-instantaneous convergence for ultrasound imaging systems deploying line arrays, circular arrays, spherical arrays of sensors or any superposition of these types of arrays.
It is yet another object of the invention to provide a 3D ultrasound imaging system comprising a multidimensional sensor array for generating high resolution images in real time using an adaptive beamforming process that is field-deployable.
In accordance with the present invention there is provided, an adaptive multidimensional beamformer having near-instantaneous convergence for ultrasound imaging systems. Implementation of the multidimensional beamformer according to the present invention provides the basis for a 3D ultrasound imaging system according to the present invention comprising a compact multidimensional sensor array and a compact processing unit that is field deployable and generates high resolution images in real time or near real time.
In accordance with the present invention there is provided, a method for multidimensional beamforming sensor time series provided by sensors deployed in a multidimensional array of an ultrasound imaging system comprising the steps of:
decomposing the multidimensional beamformer into two coherent subsets of circular array beamformers and/or line array beamformers, a first subset comprising subsequent beamformers in a first coordinate direction of the multidimensional array and a second subset comprising subsequent beamformers in a second other coordinate direction of the multidimensional array;
beamforming for a predetermined beam steering direction of data relating to the sensor time series by applying the subsequent beamformers of the first subset, each beamformer producing a beam time series; and,
beamforming for the predetermined beam steering direction each beam time series of the first subset of beamformers applying the subsequent beamformers of the second subset for the steered direction producing one beam time series for the beam steering direction.
In accordance with aspect of the present invention there is provided, a method for multidimensional beamforming sensor time series provided by sensors deployed in a multidimensional array of an ultrasound imaging system using a sub-aperture configuration, the method comprising the steps of:
a) dividing the multidimensional beamformer into a plurality of subsequent sub-apertures;
b) decomposing each sub-aperture into two coherent subsets of circular array beamformers and/or line array beamformers, a first subset comprising subsequent beamformers in a first coordinate direction of the multidimensional array and a second subset comprising subsequent beamformers in a second other coordinate direction of the sub-aperture;
c) conventional beamforming each sub-aperture for a predetermined beam steering direction a Fourier transform of the sensor time series by applying the subsequent beamformers of the first subset each beamformer producing a beam time series;
d) conventional beamforming each sub-aperture for the predetermined beam steering direction the beam time series produced by the beamformers of step c) by applying the subsequent beamformers of the second subset for the steered direction producing one beam time series for the beam steering direction for each sub-aperture; and,
e) adaptive beamforming on line arrays, each line array comprising beam time series of different subsequent sub-apertures in one coordinate direction, providing one or more beam time series for the beam steering direction.
In accordance with another aspect of the present invention there is provided, a method for beamforming sensor time series provided by sensors of an ultrasound imaging system using a coherent broad band adaptive beamformer, the method comprising the steps of:
segmenting the continuous sensor time series into a set of overlapped data sets;
calculating a FFT of each overlapped data set producing a set of Fourier transforms of the overlapped data sets for different frequency bins;
forming a cross spectral density matrix from the Fourier transforms of the overlapped data sets for each frequency bin and each predetermined steering direction;
forming a steering covariance matrix using the cross spectral density matrix and a diagonal matrix of conventional steering vectors, one steering covariance matrix for each steering direction and a frequency band of interest;
inverting the steering covariance matrices;
estimating adaptive steering vectors by assuming stationarity across frequency bins of a frequency band of interest and considering an estimate of the steering covariance matrix being the same as a narrow band estimate for a center frequency of the frequency band of interest;
determining narrow band adaptive steering weights using the estimate of the adaptive steering vectors;
forming adaptive beams in frequency domain from the Fourier transform of the overlapped data sets and the adaptive steering weights;
forming adaptive beams in time domain through IFFT; and,
determining continuous beam time series by discarding overlap and concatenation of segments.
In accordance the present invention there is further provided, a field deployable 3D ultrasound imaging system for producing high resolution 3D images of an object in real time, the ultrasound system comprising:
a source for emitting ultrasound waves;
a compact adaptive multidimensional sensor array for capturing reflections of the ultrasound waves, the ultrasound waves being reflected by different structures within the object, and for providing sensor time series indicative of the reflected ultrasound waves;
a compact processing unit for:
receiving the sensor time series produced by the multidimensional sensor array;
processing the sensor time series in order to produce continuous beamtime series by:
decomposing a multidimensional beamformer into sub-apertures comprising coherent subsets of circular array beamformers and/or line array beamformers;
conventional beamforming circular arrays;
adaptive beamforming line arrays; and,
reconstructing 3D images from the beam time series in real time; and,
a display for displaying the reconstructed 3D images in real time.